Ink-jet printers among nonimpact printers progressively extending their market in recent years are based on the simplest principle and suitable for color printing. The so-called drop-on-demand (DOD) ink-jet printers which jets ink particles only when dots are formed are major ones among ink-jet printers.
Representative head systems for DOD ink-jet printers are, for example, a Kayser head system disclosed in JP-B No. 53-12138 and a thermal-jet head system disclosed in JP-B No. 61-59914.
A Kayser ink-jet head disclosed in JP-B No. 53-12138 is difficult to down-size, and a thermal-jet ink-jet head disclosed in JP-B No. 61-59914 has a problem that the ink burns and sticks to the ink-jet head because intense heat is applied to the ink.
An ink-jet head proposed to overcome both the foregoing disadvantages employs piezoelectric elements having a piezoelectric strain constant d.sub.33 (hereinafter referred to as "d.sub.33 mode ink-jet head").
The d.sub.33 mode ink-jet head employs thin pieces of a piezoelectric material (piezoelectric elements). Electrodes are formed on the opposite surfaces of the piezoeletric element, and the piezoelectric element is polarized in the direction of an electric field created between the electrodes so that the piezoelectric element has the piezoelectric strain constant d.sub.33. When an electric field is created across the electrodes, the piezoelectric element expands and contracts in the direction of the thickness (the d.sub.33 direction) to pressurize an ink chamber.
Known d.sub.33 mode ink-jet heads are disclosed in JP-A Nos. 3-10845 and 3-10846.
FIGS. 11 and 12 show a structure of the inkjet head disclosed in JP-A No. 3-10846.
The ink-jet head disclosed in JP-A No. 3-10846 comprises a cover block 211 provided with two recesses, and a piezoelectric element block 213 which expands and contracts in the direction of the thickness (the d.sub.33 direction) when a voltage is applied thereto.
The piezoelectric block 213 has a layered structure. The piezoelectric block 213 is made of lead titanate zirconate. The piezoelectric block 213 is provided with grooves 216a, 216b, 216c and 216d extending perpendicularly to the paper. A portion of the piezoelectric block 213 between the grooves 216aand 216b is a first driving piezoelectric element 217a. The first driving piezoelectric element 217a is provided with a first electrode 215a. A portion of the piezoelectric block 213 between the grooves 216c and 216d is a second driving piezoelectric element 217b. The second driving piezoelectric element 217b is provided with a second electrode 215b.
The two recesses in the cover block 211 are covered with an oscillation plate 212. One of the recesses in the cover block 211 and the oscillation plate 212 define a first ink chamber 218a. The other recess in the cover block 211 and the oscillation plate 212 define a second ink chamber 218b. The first ink chamber 218a is connected to a first nozzle 219a. The second ink chamber 218b is connected to a second nozzle 219b.
In this ink-jet head, when a voltage is applied to, for example, the first electrode 215a, the first driving piezoelectric element 217a expands in the direction of the thickness (the direction d.sub.33). Consequently, the oscillation plate 212 is bent in the same direction to pressurize the first ink chamber 218a, whereby an ink particle is jetted through the first nozzle 219a.
The prior art ink-jet head disclosed in JP-A No. 3-10845 is substantially the same in principal constitution as the ink-jet head disclosed in JP-A No. 3-10845.
The foregoing prior art ink-jet head has the following problems.
As is obvious from FIGS. 11 and 12, the respective front and back surfaces of the piezoelectric block 213, and the electrodes 215a and 215b are exposed, and the open ends of the nozzles 219a and 219b are flush with the front end surface. Therefore, there is the possibility that the ink leaked from the nozzles 219a and 219b spreads over the front and back surfaces of the piezoelectric block 213, and the electrodes 215a and 215b to short the electrodes 215a and 215b. Particularly, since the distance between the electrodes 215a and 215b is very short in a piezoelectric block of a layered structure, it is possible that breakdown between the electrodes is caused by moisture contained in the atmosphere in an environment of high humidity, which causes a problem in the safety of operation.
An apparatus, such as disclosed in JP-A No. 4-77669, which jet a liquid, such as ink, through fine nozzles closes the nozzles by pressing a cap against the front ends of the nozzles while the nozzles are not used to prevent the clogging of the nozzles due to the drying of the ink remaining in the nozzles, and is provided with a cleaning mechanism having a blade for wiping off the liquid leaked through the nozzles. It is preferable that the ink-jet head is provided with such a cap and a cleaning mechanism.
However, when the front end surfaces of the piezoelectric block 213 and the electrodes 215a and 215b are exposed in a plane flush with the open ends of the nozzles 219a and 219b, it is possible that the ink flows along the cap and the blade and adheres to the end surfaces of the piezoelectric block 213 and the electrodes 215a and 215b to cause breakdown between the electrodes 215a and 215b.
Such a problem may be solved by shifting the front end surfaces of the piezoelectric block 213 and the electrodes 215a and 215b from a position corresponding to a plane including the open ends of the nozzles 219a and 219b, which, however, makes only the front surface of the cover block 211 to be subjected to the pressure of the cap and the frictional force of the cleaning blade.
Consequently, it is highly possible that the cover block 211 is distorted and damaged when the cap is brought into contact with the front surface of the cover block repeatedly and the cleaning blade rubs the front surface repeatedly. The cover block 211 is provided with the nozzles 219a and 219b through which ink particles are jetted, and ink particles will be jetted in wrong directions even if the cover block 211 is distorted even slightly, and thereby print quality is deteriorated.
In the foregoing prior art inkjet head, the driving piezoelectric elements 217a and 217b are supported by nondriven portions (portions 217c in FIGS. 11 and 12) of the piezoelectric block 213. Since the piezoelectric block 213 of a layered structure is fabricated by alternately laminating layers of lead titanate zirconate and electrode films, forming the grooves 216a, 216b, 216c and 216d to space apart the driving piezoelectric elements 217a and 217b and the nondriven portions 217c, the nondriven portions 217c have electrode films 215c.
Accordingly, when reaction force resulting from the distortion of the driving piezoelectric elements 217a and 217b is sustained only by the nondriven portions 217c, there is the possibility that the nondriven portions 217c are unable to withstand the reaction force and the inkjet head is broken up.